Gene flow between Clearfield™ rice and red rice

Imidazolinone-herbicide-resistant Clearfield™ (CL) rice allows the selective chemical control of red rice (Oryza sativa L.), a major weed problem in rice-growing areas of the southern US, using herbicides that otherwise are toxic to the crop. However, selection imposed by the herbicide of resistant individuals resulting from gene flow, and of naturally tolerant individuals, is a major concern. Experiments were conducted at the Rice Research and Extension Center, Stuttgart, AR, US in 2002–2004 to evaluate the effects of cultivar, distance from pollen source, and planting date on natural outcrossing between CL rice and Stuttgart strawhull red rice. The experiments were planted in a split-plot design with planting date (April and May) as main plot and cultivar (CL121 and CL161) as subplots. An encircle-population combination technique was used to determine distance from the rice pollen source at which hybrids were detected. Resistance of the F1 plants was determined by repeated spraying of imazethapyr, and DNA analysis using SSR primer RM 180 to confirm hybrids. All resistant red rice plants were confirmed hybrids. Outcrossing was higher with CL161 (0.008%) than with CL121 (0.003%) regardless of planting date. Averaged over cultivars, outcrossing rate between April (0.004%) and May (0.006%) plantings did not differ. The majority of hybrids were located within 1 m from CL rice. However, a few hybrids were found up to 6 m from the CL rice, which was the limit of detection in these experiments. All F1 hybrids were taller and had longer flag leaves than their parents. F1 hybrids had rough texture and pale-colored leaves similar to the red rice parent. Gene flow between CL rice and red rice is a concern, and even at the low outcrossing rates in these experiments, approximately 170 resistant plants/ha resulted.